1. Field of the Invention
The present invention relates to a semiconductor device using an organic semiconductor and a manufacturing method thereof.
2. Description of the Related Art
In recent years, a study of an organic transistor using an organic semiconductor has been actively carried out. It is expected that a thin film is easily formed and that an organic semiconductor device is formed over a flexible substrate such as plastic, a film substrate, or a paper which is lightweight with flexibility by making the most of the characteristic of an organic semiconductor. Further, an organic semiconductor material for forming a channel is extremely inexpensive and low in deposition temperature, and a process without using a vacuum chamber such as an ink-jet method, a printing method, a stamp method, or the like can also be applied. Therefore, it is expected to drastically reduce the manufacturing cost of a general semiconductor device using an organic transistor.
An organic transistor is composed of a glass substrate, a gate electrode, a gate insulating layer, source and drain electrodes, and an organic semiconductor layer. A structure where source and drain electrodes are provided below an organic semiconductor layer is referred to as a bottom contact structure and a structure where a source and drain electrodes are provided over an organic semiconductor layer is referred to as a top contact structure.
In the case of an organic transistor, carrier mobility can be increased by employing the top contact structure. However, it is difficult to use a step such as photolithography to perform microfabrication of pattern or the like in this structure; thus, microfabrication may be limited to the bottom contact structure where an organic semiconductor layer is formed after forming source and drain electrodes. Therefore, the structure of an organic transistor needs to be applied in accordance with an advantage and a disadvantage of each structure.
It is known that carrier mobility of an organic transistor largely depends on morphology (an amorphous, polycrystalline, single crystalline state, or the like) of an organic semiconductor. In particular, an organic transistor using a single crystal of an organic semiconductor layer shows high carrier mobility and has carrier mobility almost the same as that of amorphous silicon. A liquid phase growth method, a vapor-phase transport method, or the like can be given as a method for obtaining a favorable single crystal of an organic material.
In order to improve characteristics by actively using a single crystal for an organic transistor, a technique for selectively coating a position where a single crystal is made to grow is necessary. In coating a single crystal in such a manner, such a method as wettability of a substrate portion is reformulated to selectively grow a single crystal is typically used.
As an example where a single crystal of an organic semiconductor layer is used, a structure where an island-shaped projection layer in which island-shaped projections having low surface energy are formed by being dispersed is provided over a surface of a gate insulating layer is suggested in order to suppress a crystalline state of a pentacene deposited film and to provide an organic semiconductor element having high carrier mobility with low voltage driving (for example, see Reference 1: Japanese Patent Application Laid-Open No. 2004-23021).
In addition, an organic transistor the characteristic of which is to form a fluorine polymer layer over a surface of a gate insulating layer is suggested by limiting a contact angle of deionized water with respect to the surface of a gate insulating layer to 50° C. or more and 120° C. or less (for example, see Reference 2: Japanese Patent Application Laid-Open No. 2001-94107).
When a single crystal of an organic semiconductor layer is formed as in the above Reference 1, an interface between an organic semiconductor of a channel where carriers are spread out and a gate insulating layergate insulating layer looses its planarity; therefore, it is hard to say that carrier mobility of a single crystal of an organic semiconductor layer is utilized sufficiently.
In addition, in the above Reference 2, a yield is to deteriorate when a single crystal of an organic semiconductor layer cannot be grown through the entire channel.